1. Field of the Invention
This invention relates to an image forming apparatus such as a copier or a printer using the electrophotographic process or the electrophotographic recording process, and particularly to an image forming apparatus using the contact transfer process.
2. Related Background Art
An image forming apparatus of the transfer type in which a transferable image of image information which is generally a toner image is formed by a suitable image forming process and is borne on the surface of an image bearing member, the transferable image is transferred to a transfer material such as paper, the image is fixed, the transfer material is outputted as an image forming material (a copy or a print) and the image bearing member is repetitively used for image formation has been widely put into practical use.
FIG. 8 of the accompanying drawings is a schematic construction model view of a typical example of such an image forming apparatus of the transfer type. This example of the image forming apparatus is a copier or a printer utilizing the transfer type electrophotographic process.
The reference numeral 10 designates a drum type electrophotographic photosensitive member (hereinafter referred to as the photosensitive drum) as an image bearing member, and it is rotatively driven at a predetermined peripheral speed (process speed) in the clockwise direction of arrow X, and the image forming process of charging, image exposing, developing, transferring and cleaning is applied to this photosensitive drum.
That is, the rotatively driven photosensitive drum 10 has its surface uniformly charged to a predetermined polarity and potential by a primary charger 11.
The charged surface is then subjected to image exposure 12 by image exposing means (such as an original image projecting and exposing device or an image modulated laser beam scanning and exposing device), not shown, as image information writing means, whereby the charging potential of an exposed light portion is attenuated and an electrostatic latent image corresponding to exposed image information is formed on the surface of the photosensitive drum.
The electrostatic latent image is sequentially made into a visible image as a transferable toner image (visualized image) in a developing region A by a developing device 13.
The toner image is transferred to a transfer material (transfer paper) 18 in a transfer region T by transfer means.
The transfer means in the present example is transfer means of the contact transfer type using a roller-shaped contact transfer charger 16 (hereinafter referred to as the transfer roller).
The transfer roller 16 comprises, for example, a mandrel and an elastic layer of medium resistance formed around the mandrel, and is urged against the photosensitive drum 10 with a predetermined pressure force against the elasticity of the elastic layer to thereby form a transfer region T (a transfer nip portion), and is rotated at substantially the same peripheral speed as the rotational peripheral speed of the photosensitive drum in a forward direction relative to the rotation of the photosensitive drum 10.
The transfer material 18 is fed from a feeding means portion, not shown, and is timed by registration rollers 15 disposed on this side of the transfer region T and is fed to the transfer region T.
That is, the registration rollers 15 feed the transfer material 18 to the transfer region T so that the leading end portion of the transfer material 18 may just come to the transfer region T when the leading end portion of the toner image area formed on the surface of the rotating photosensitive drum 10 comes to the transfer region T.
The transferred material 18 fed to the transfer region T is pinched and conveyed through the transfer region T with its surface being in close contact with the rotating photosensitive drum 10. Also, during the time from after the leading end portion of the transfer material 18 has come to the transfer region T until the trailing end portion thereof leaves the transfer region T, a predetermined transfer bias is applied from a transfer bias voltage source 20 to the mandrel of the transfer roller 16.
In the process in which the transfer material 18 is pinched and conveyed through the transfer region T, the toner image on the rotating photosensitive drum 10 is sequentially transferred to the transfer material 18 by the action of a transfer electric field formed by the transfer roller 16 as a contact transfer charger and a pressure force in the transfer region T.
The transfer material 18, when it leaves the transfer region T, is separated from the surface of the rotating photosensitive drum 10 and is conveyed to an image fixing device, not shown, and the transferred toner image is fixed as a permanent image on the surface of the transfer material, and the transfer material is discharged as an image forming material (a copy or a print).
The surface of the photosensitive drum 10 after the separation of the transfer material is cleaned by a cleaner 17 with any residual toner and other contaminants such as paper powder thereon removed, and is repetitively used for image formation.
As the image forming methods, there are, for example, a normal (regular) developing method of exposing the charged surface of the photosensitive member corresponding to the background portion of image information (a background exposing method), and developing the portion other than the background portion, and a reversal developing method of exposing the charged surface of the photosensitive member corresponding to the image information portion (an image exposing method), and developing a non-exposed portion, and these methods are used so as to make the most of their respective features.
Transfer means of the contact transfer type using the contact transfer charger typified by the transfer roller 16 as described is popular as transfer means for transferring the toner image as a transferable image on the photosensitive drum 10 as an image bearing member onto the transfer material 18 such as paper, and as compared with transfer means using a corona charger of the non-contact type, this has merits such as the small capacity of the power source and the small amount of production of a discharge product typified by ozone.
Now, when use is made of the transfer means of the contact transfer type as previously described, there are known two methods, i.e., a "constant current control" method of setting a transfer current to a predetermined value, and a "constant voltage control" method of setting a transfer voltage to a predetermined value.
In the case of the constant current control method, however, when a thick and small-sized transfer material such as a postcard or an envelope is supplied, the transfer current is liable to flow to a non-paper supply area from the difference in resistance between a paper supply area and the non-paper supply area during paper supply, and the amount of transfer charges imparted to the back of the transfer material becomes small, and bad transfer such as transfer void is liable to occur. This phenomenon is more remarkable as the volume resistance of the transfer roller 16 which is a contact charger is smaller and therefore, it can be improved by making the volume resistance of the transfer roller higher, but there arise such problems as the higher voltage of the power source and transfer leak.
In contrast with this, in the case of the constant voltage control method, there are not the problems as noted above, but yet there are the following problems.
That is, this method is liable to be affected by the unevenness of the resistance value of the transfer roller 16. In order to prevent this, use has heretofore been made of a method called ATVC (Active Transfer Voltage Control: Japanese Patent Application Laid-open No. 2-123385) in which a predetermined constant current is applied to the transfer roller before image formation, and the resistance value of the transfer roller is judged from the then power source output voltage to thereby determine a constant voltage control value.
However, when particularly thin and large-sized paper is used, as compared with the case where the aforedescribed constant current control method is used with respect to the unevenness of the amount of transfer current imparted to the back of the transfer material, the case where the constant voltage control method is used is apparently inferior.
As described above, the constant current control method and the constant voltage control method have their own merits and demerits. In order to solve this, Japanese Patent Application Laid-open No. 9-297476 proposes a method of selecting one of constant current control and constant voltage control in conformity with environmental conditions.
Also, Japanese Patent Application Laid-open No. 8-114989 proposes a method of effecting constant current control when the resistance value of the transfer roller is higher than a reference value, and effecting constant voltage control when the resistance value of the transfer roller is lower than the reference value.
However, even these prior-art methods still leave the following problems to be solved.
That is, even when the thickness of the transfer material was changed, the constant current control or the constant voltage control was selected depending on the environmental conditions and the resistance value of the transfer roller, and depending on the thickness of the transfer material used, the transfer charges of the back of the transfer material became deficient to cause transfer void, or conversely the transfer voltage was too strong and memory was sometimes produced in the photosensitive drum as an image bearing member.
Particularly, in a method of forming a toner image on the surface of a photosensitive member holding potential of the same polarity as that of the toner which is typified by reversal development, the transfer polarity during transfer is opposite to the polarity of the surface of the photosensitive member, and as compared with the normal developing method, drum memory was liable to be produced.